In most hydrometallurgical processes for the recovery of gold, elemental gold is solubilized by oxidation to ionic gold and complexation with cyanide. While the use of cyanide for gold recovery systems has been the primary process for the recovery of gold for many decades, the toxicity of cyanide makes introduction into groundwaters associated with gold ores highly undesirable. Apart from the toxicity of cyanide to man, its use poses long-term environmental and waste disposal problems.
Consequently, the use of iodide as a complexing agent in the gold recovery process carries with it significant benefits. Direct benefits are derived from the lack of immediate pollution of the leach area with a highly toxic material, such as cyanide. In addition, the reduced requirements of containment and disposal of leach waste materials are highly economic. This is particularly true of in-situ leach processes where such factors are of great concern in determining the economic feasibility of gold recovery.
While iodine leach processes are well known in the art, a significant problem with prior methods is that they suffer from inefficient recovery and reuse of iodine. Much of the iodine is consumed by the ore body. Since iodine is only 300 times more abundant than gold itself, this relative scarcity can make the use of iodine too expensive for commercially economic recovery of gold via in-situ leaching processes.
Another problem with these processes concerns the solubility of iodine in a typical aqueous system. Alkali metal cations, such as sodium and potassium, generally must be added to promote dissolution. This addition of foreign cations generates a substantial portion of the cost associated with waste handling and restoration of the mining area, and is therefore undesirable.
Finally, while some gold recovery methods allowing the use of iodine employ reoxidation of the iodine in order to conserve the iodine for reuse, these methods typically require the introduction of oxidizing anions such as chlorine. This is also undesirable for both the ecological and economic reasons discussed above with respect to the addition of cations.
The inventor's process overcomes these problems with prior known methods. The instant process comprises a practical mining technique for the recovery of gold from permeable gold ores which does not require the introduction or use of toxic species such as cyanide. Moreover, the method allows the use of iodine in the leach process without the addition of cations into the lixiviant in order to promote dissolution of iodine during its introduction into the leach. Finally, the process permits reoxidation of the iodide to iodine, thus conserving the starting material, without the introduction of oxidizing anions, such as those of chlorine, chlorate, or hypochlorite. Thus the costs associated with environmental clean up of toxic materials, and with the exhaustion and consumption of scarce iodine are greatly mitigated.